Disclosed herein is a method and an apparatus for obtaining homogeneous ink for inkjet devices, with equipment for generating an ink jet, with a nozzle arrangement comprising an ultrasonic oscillator and a nozzle for dividing the ink jet into individual equally large ink droplets, with a charge tunnel with which at least some of the ink droplets are provided with an electric charge, with a deflection device with which the individual electrically charged ink droplets are deflected, and with a homogenization droplet catcher.
In continuous inkjet printers (CIJ printers), an ink jet 12 (see FIG. 1) emerges at pressure from the printhead 10 via a nozzle. This jet 12 is modulated via a piezoelectric transducer which is located behind the nozzle, with the result that a uniform breakup into individual droplets 16 is achieved (Rayleigh's droplet breakup). The detaching droplets 16 are electrostatically charged to a greater or lesser degree via a charge tunnel 18. The 10 to 40 m/s fast droplets 16 then fly through a larger deflection electrode 20, where they are deflected laterally or vertically by different, specific electric charge states. Depending on the type of device, the charged or uncharged droplets 16 now reach the surface 21 to be printed on. Droplets 16 that are not needed are already deflected at the printhead in a customary droplet catcher 22, collected and fed to the ink system again. It is known from European Patent Document No. 0 362 101 to inspect and control the speed of the droplets, the quality of the ink and the formation and charging of the droplets in order to achieve a high print quality.
An inkjet matrix printer (CIJ printer) with two gutters is known from German Patent Document No. OS 23 31 803. The first gutter generates control signals for the synchronization of the droplet formation and the charging of the droplets. At inspection intervals, the second gutter collects unused droplets, which have a very high charge compared with the droplets used for the printing, whereby system errors, such as errors in the deflection voltage or in the font size, are detectable.
In the abstract of Japanese Patent Document No. 56 113 463 A, a two-part droplet catcher for a CIJ printer is described which collects undeflected droplets and droplets which have the opposite charge to the droplets that are used for the writing. These oppositely charged droplets are used to determine the ink viscosity.
Special inks are used in CIJ printers. These inks are composed of dyes, binders and solvents. According to requirements, additional salts, quaternary ammonium compounds or other agents can be contained in order to increase the conductivity of the ink. Moreover, adhesion promoters, as well as agents for increasing or lowering the surface tension, can be contained. Besides dyes, pigments can also be used to colour the ink. While dye inks produce more brilliant colours by comparison, pigment inks display the advantage that they run less on the surface to be printed on and are more true and display higher contrast.
It is particularly important in the CIJ printing process that the ink is as homogeneous as possible, in order that ink droplets that are as uniform as possible form. The ink droplets are to have a consistent droplet breaking length, droplet speed, mass and electrical chargeability. The homogeneity of the ink is a prerequisite for being able to divide the ink jet into small droplets with constant chemical and physical properties. In particular, the chargeability in relation to the weight is decisive here, because only when the droplets have a particular charge/mass ratio can they be directed onto their allotted place in the writing matrix. A non-uniform droplet formation therefore leads to poorly controllable or straying ink droplets, which result in a deterioration of the typeface of the printhead.
In order to produce inks with as high as possible a degree of homogenization, care is conventionally taken that the individual components of the ink have as high as possible a solubility and dispersibility, and procedural means which result in as high a homogeneity of the ink as possible are chosen. In particular, the ink is filtered several times during the production. Moreover, until now, the ink has been matched in each case precisely to the device in which the ink is to be used (see European Patent Document No. 0 438 427).
The poorer the quality of the ink used is, the more difficult the adjustment of the printhead proves to be. Ink with a deficient quality leads to an acceptable print result only in the case of an exactly adjusted printhead. This can have the result that the print result deteriorates drastically even in the case of a slight change in the ink consistency or a variation of the ambient conditions. In contrast, ink with optimum quality can be used in a wide adjustment range without resulting in an impairment of the print image.